Exemplary embodiments relate to the page buffer of a nonvolatile memory device and a method of performing a program verification operation using the same and, more particularly, to the page buffer of a nonvolatile memory device and a method of performing a program verification operation using the same, which are capable of reducing the time that it takes to perform a program verification operation.
Recently, there has been an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and which do not require the refresh function of rewriting data at specific intervals. Furthermore, to increase the degree of integration, attempts are being made to store data of more than one bit in a single cell.
A nonvolatile memory device capable of storing data of plural bits in one cell as described above is called a multi-level cell (MLC) nonvolatile memory device. For example, a MLC nonvolatile memory device capable of storing 2 bits can store four different data, such as 11, 10, 01, and 00, in one cell. Accordingly, the degree of integration can be increased from a logical viewpoint.
A nonvolatile memory device typically includes a memory cell array in which cells for storing data are arranged in a matrix form and a page buffer configured to write data into a specific cell of the memory cell array or to read data stored in a specific cell thereof. The page buffer includes pairs of bit lines coupled to the memory cells, a register configured to temporarily store data to be written into the memory cell array or to temporarily store data read from a specific cell of the memory cell array, a sense node configured to sense the voltage level of a specific bit line or a specific register, and a bit line selection unit configured to control the connection between the specific bit line and the sense node.
The above-described nonvolatile memory device may form a greater number of distributions of the threshold voltages in order to store a plurality of data in one memory cell during a program operation. For example, to store data of 3 bits in one memory cell, eight distributions of the threshold voltages must be implemented. To this end, during the program operation, program voltages corresponding to respective data values are supplied to the gate of the corresponding memory cell so that the memory cell has distributions of the threshold voltages corresponding to the respective data values.
To perform a program verification operation after the program operation, a sense operation has to be performed using different verification voltages, supplied to a word line, corresponding to the respective distributions of the threshold voltages. Accordingly, an MLC device capable of storing data of 3 bits requires at least eight verification operations. Consequently, as the number of data to be stored in one memory cell is increased, the number of program verification operations and the time that it takes to perform verification increases.